Greater China is incredibly diverse with at least 55 ethnic tribes in addition to the Han majority (which comprises 92% of the population of the mainland China, 98% of the population of Taiwan, 74% of the population of Singapore, 24.5% of population of Malaysia, and about 20% of the entire human population.)
Apparently, the Yayoi people http://en.wikipedia.org/wiki/Yayoi_period who invaded (?) and settled Japan in 300 B.C.E. http://en.wikipedia.org/wiki/3rd_century_BC can be traced genetically to China's coastal Jiangsu province area http://www.trussel.com/prehist/news111.htm but their full migratory history is yet to be determined. http://en.wikipedia.org/wiki/List_of_Neolithic_cultures_of_China The rice-farming Yayois supplanted/absorbed the semi-sedentary hunter-gatherer Jomon people who had been thriving in the Japanese archipelago since around 11,000 B.C.E. or earlier http://www.ncbi.nlm.nih.gov/pubmed/18484628 , making this event seemingly as significant as the much earlier one where homo sapiens sapiens co-existed with the Neanderthals who eventually disappeared. http://en.wikipedia.org/wiki/Neanderthal_interaction_with_Cro-Magnons
According to DNA analysis, the Japanese genetic makeup varies somewhat from that of the contemporary Han Chinese (although some shared genetic markers point to migration through mainland China.) Furthermore, the Yayoi people who settled in Japan don't seem to have been Han Chinese whose political power was still concentrated north of the Yangtze River. Also, the Japanese language is linguistically an isolate, not a Sinitic language and resembling syntactically the contemporary Korean, Turkish and Hungarian.
Although Chinese culture influenced Japan considerably http://en.wikipedia.org/wiki/Asuka_period , Korean culture exerted some influence also, the records of which exist from around the 6th century when Buddhism was introduced to Japan through the peninsula. http://en.wikipedia.org/wiki/Korean_influence_on_Japanese_culture Notably, a son of a Korean king settled in Japan and his descendants were eventually assimilated into the social fabric of Japan with one female descendant becoming a concubine of an imperial Yamato prince and giving birth to the future Emperor Kammu over a hundred years later. http://en.wikipedia.org/wiki/Kudara_no_Konikishi_clan There were other clans founded by Korean and Chinese immigrants. http://en.wikipedia.org/wiki/Hata_clan http://en.wikipedia.org/wiki/Wani_%28scholar%29
Little seems to be known of the history between 300 B.C.E. and early 6th century, the earliest example of the written language (simplified version of Chinese ideograms) being from the year 471 C.E. http://en.wikipedia.org/wiki/Hiragana Are the Ainus http://en.wikipedia.org/wiki/Ainu_people descendants of Jomon people who fled the Yayoi invasion or descendants of both Jomon and Yayoi like the rest of the population in Japan? http://genome.cshlp.org/content/14/10a/1832.long What about race? Had the Japanese been isolated long enough to develop a genetic homogeneity of a distinct race? Apparently, not long enough. http://www.pbs.org/race/000_About/002_04-background-01-02.htm
It could be said that the Japanese civilization got somewhat of a late start evolving as a culture on par with other civilized cultures around the world but caught up fairly quickly to embrace a social reality of Japanese-ness with multiple substrate cultures: Jomon, Yayoi and Yamato.